The prognostic significance of DNA aneuploidy was studied retrospectively in 177 Stage I cutaneous melanomas. DNA content was determined by flow cytometry of propidium iodide-stained nuclei recovered from formalin-fixed, paraffin-embedded material. Of 162 evaluable histograms, 124 were diploid, 35 aneuploid, and 3 tetraploid. Aneuploidy strongly correlated with established predictors of unfavorable prognosis, namely, thickness p < .005, level p < 0.005, ulceration p < 0.005, and presence of vertical growth phase p < 0.02. Overall, aneuploidy was strongly correlated with recurrence (p < 0.005) and shorter disease-free survival (p < 0.0001). Aneuploidy was an independent predictor of recurrence for tumors < 1.5 mm thick (p < 0.0001) and ≥ 3 mm thick (p = 0.031). Four melanomas 1.5-2.9 mm thick, aneuploid tumors had a 27% higher recurrence rate than diploid tumors (63% vs. 36%). This was not statistically significant (p = 0.247). In a multivariate analysis of common predictors stratified by thickness, DNA aneuploidy was the most significant independent parameter (p < 0.002). DNA content appears to be an important stratification parameter for Stage I cutaneous melanoma.